It is important to determine the location of a mobile telephone or other mobile device capable of radio communication especially in an emergency situation. One method of assessing geolocation of a mobile device is utilizing the mobile device in conjunction with a geolocation system. Exemplary geolocation systems include satellite navigation systems. For example, one Global Navigation Satellite System (GNSS) is the NAVSTAR Global Positioning System (i.e., GPS). GPS is a radio positioning system providing subscribers with highly accurate position, velocity, and time (PVT) information.
FIG. 1 is a schematic representation of a constellation 100 of GPS satellites 101. With reference to FIG. 1, GPS includes a constellation of GPS satellites 101 in non-geosynchronous orbits around the earth. The GPS satellites 101 travel in six orbital planes 102 with four of the GPS satellites 101 in each plane. Of course, a multitude of on-orbit spare satellites may also exist. Each orbital plane has an inclination of 55 degrees relative to the equator. In addition, each orbital plane has an altitude of approximately 20,200 km (10,900 miles). The time required to travel the entire orbit is just under 12 hours. Thus, at any given location on the surface of the earth with clear view of the sky, at least five GPS satellites are visible at any given time.
With GPS, signals from the satellites arrive at a GPS receiver and are utilized to determine the position of the receiver. GPS position determination is made based on the time of arrival (TOA) of various satellite signals. Each of the orbiting GPS satellites 101 broadcasts spread spectrum microwave signals encoded with satellite ephemeris information and other information that allows a position to be calculated by the receiver. Presently, two types of GPS measurements corresponding to each correlator channel with a locked GPS satellite signal are available for GPS receivers. The two carrier signals, L1 and L2, possess frequencies of 1.5754 GHz and 1.2276 GHz, or wavelengths of 0.1903 m and 0.2442 m, respectively. The L1 frequency carries the navigation data as well as the standard positioning code, while the L2 frequency carries the P code and is used for precision positioning code for military applications. The signals are modulated using bi-phase shift keying techniques. The signals are broadcast at precisely known times and at precisely known intervals and each signal is encoded with its precise transmission time.
GPS receivers measure and analyze signals from the satellites, and estimate the corresponding coordinates of the receiver position, as well as the instantaneous receiver clock bias. GPS receivers may also measure the velocity of the receiver. The quality of these estimates depends upon the number and the geometry of satellites in view, measurement error and residual biases. Residual biases include satellite ephemeris bias, satellite and receiver clock errors and ionospheric and tropospheric delays. If receiver clocks were perfectly synchronized with the satellite clocks, only three range measurements would be needed to allow a user to compute a three-dimensional position. This process is known as multilateration. However, given the expense of providing a receiver clock whose time is exactly synchronized, conventional systems account for the amount by which the receiver clock time differs from the satellite clock time when computing a user's position. This clock bias is determined by computing a measurement from a fourth satellite using a processor in the receiver that correlates the ranges measured from each satellite. This process requires four or more satellites from which four or more measurements can be obtained to estimate four unknowns x, y, z, b. The unknowns are latitude, longitude, altitude and receiver clock offset. The amount b, by which the processor has added or subtracted time is the instantaneous bias between the receiver clock and the satellite clock.
However, the signal received from each of the visible satellites does not necessarily result in an accurate position estimation. The quality of a position estimate largely depends upon two factors: satellite geometry, particularly, the number of satellites in view and their spatial distribution relative to the user, and the quality of the measurements obtained from satellite signals. For example, the larger the number of satellites in view and the greater the distances therebetween, the better the geometry of the satellite constellation. Further, the quality of measurements may be affected by errors in the predicted ephemeris of the satellites, instabilities in the satellite and receiver clocks, ionospheric and tropospheric propagation delays, multipath, receiver noise and RF interference. With standalone GPS navigation or geographic location, where a user with a GPS receiver obtains code-phase ranges with respect to a plurality of satellites in view, without consulting with any reference station, the user is very limited in ways to reduce the errors or noises in the ranges.
One method and apparatus to eliminate erroneous GPS signals is disclosed by copending U.S. application Ser. No. 11/405,404, filed Apr. 18, 2006 by the inventors hereof, entitled, “Method and Apparatus for Geolocation Determination,” the entirety of which is herein incorporated by reference. This invention compares predicted C/A chips with measured chips and discards satellite signals having significant inconsistencies. However, pruning erroneous GPS signals based on code phase prediction may not be necessary if no erroneous signals exist, and such a technique may require a reasonably accurate cell database.
Accordingly, there is a need for a method and apparatus for geographic location determination of a device that would overcome the deficiencies of the prior art. Therefore, an embodiment of the present subject matter provides a method for determining the location of a device. The method comprises the steps of receiving a first plurality of signals comprising one signal from each of a first plurality of satellites and determining a first location of the device as a function of the first plurality of signals. If the first location is not within a predetermined threshold then the method comprises the step of determining a second location of the device as a function of a second plurality of signals wherein the second plurality of signals is a first subset of the first plurality of signals. An alternative embodiment may further comprise the step of determining a third location of the device as a function of a third plurality of signals if the second location is not within the predetermined threshold, wherein the third plurality of signals is a second subset of the first plurality of signals.
In another embodiment of the present subject matter a method is provided for determining the location of a device receiving signals from each of a plurality of satellites, the device having determined a first location from the plurality of satellite. The method comprises the steps of comparing a quality of the first location of the device with a predetermined threshold and determining a second location of the device from a first subset of the received signals if the quality of the first location is not within the predetermined threshold. Additional embodiments may further comprise the step of determining a third location of the device as a function of a second subset of the received signals if the quality of the second location is not within the predetermined threshold.
In yet another embodiment of the present subject matter a method is provided for determining the location of a device. The method comprises the steps of receiving a plurality of signals from a plurality of satellites, generating estimates of a location of the device using combinations of the plurality of signals, and selecting an estimate as defined by a quality of each of the combinations.
An alternative embodiment of the present subject matter provides an apparatus comprising a receiver for receiving a first plurality of signals comprising one signal from each of a first plurality of satellites and a means for determining a first location of the apparatus as a function of the first plurality of signals. The apparatus may further comprise a means for determining a second location of the apparatus as a function of a second plurality of signals if the first location is not within a predetermined threshold, wherein the second plurality of signals is a first subset of the first plurality of signals. Additional embodiments of an apparatus according to the present matter may further comprise a means for determining a third location of the apparatus as a function of a third plurality of signals if the second location is not within the predetermined threshold, wherein the third plurality of signals is a second subset of the first plurality of signals.
An additional embodiment of the present subject matter provides an apparatus for determining location from signals received from a plurality of Global Navigation Satellite System (“GNSS”) satellites. The apparatus comprises a receiver for receiving signals from each of a plurality of satellites, a means for determining a first location of the device as a function of a quality of the received signals, and a means for determining a second location of the device as a function of a subset of said received signals if said quality is not within a predetermined threshold.
In still another embodiment of the present subject matter a method is provided for calculating the position of a device. The method comprises the steps of receiving a first plurality of observations from a first plurality of satellites and determining a first position of the device as defined by a quality of the first plurality of observations. If the quality of the first plurality of observations fails to meet the predefined threshold then the method comprises the steps of determining a second position of the device as defined by a quality of a second plurality of observations, the second plurality being a subset of the first plurality of observations. If the quality of the second plurality of observations fails to meet the predefined threshold then additional positions of the device are determined utilizing incrementally decreasing subsets of observations until a predetermined criteria is achieved.
These embodiments and many other objects and advantages thereof will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the embodiments.